1. Field of the Invention
The present invention relates to a polyisobutene composition, a process for the preparation of polyisobutene and a process for reducing the viscosity of polyisobutenes.
2. Description of the Background
Polyisobutenes having a maximum of the molecular weight distribution of from 3 500 to 8 000 Dalton (corresponding roughly to a number average molecular weight of from 1 900 to 4 000) and preferably a high content of terminal vinylidene groups are desirable intermediates for the preparation of additives for lubricants and fuels. They are prepared by Lewis acid-catalyzed polymerization of isobutene. The Lewis acid catalyst used is frequently boron trifluoride, generally in combination with a compound capable of forming a complex with boron trifluoride. The polymerization is carried out in general in the presence of an inert diluent, such as n-hexane. As soon as the desired degree of polymerization has been reached, the catalyst is removed and/or deactivated and unconverted isobutene and the inert diluent are removed by being distilled off. Such a process is disclosed, for example, in U.S. Pat. No. 5,408,018.
The prior patent applications DE 199 48 947.5, DE 199 52 031.3, DE 199 52 030.5, DE 100 28 585.6 and DE 100 35 298.7 relate to improvements or advantageous embodiments of such processes.
In the preparation of polyisobutenes having a maximum of the molecular weight distribution of from 3 500 to 8 000, the high viscosity of these polyisobutenes presents problems when the unconverted isobutene and/or the inert diluent are distilled off. Thus, these polyisobutenes form extremely viscous boundary layers on the walls of containers or pipelines, which act as thermal insulator and hinder heat transfer. In order to distill off the final amounts of unconverted isobutene and/or inert diluent, it is therefore necessary to choose a high heating temperature, although there is the danger of thermal damage to the polyisobutene. In addition, the high viscosity of the material being distilled leads to the formation of a stubborn froth which adversely affects the quality of the distillative separation.
The viscosity of the polyisobutenes, which is high even at elevated temperatures, increases even further on cooling so that products which can scarcely be handled mechanically, i.e. can no longer be pumped and conveyed, are obtained at room temperature.